In 3GPP (3rd Generation Partnership Project), which is a project aiming to standardize a mobile communication system, attempts are being made to enhance LTE (Long Term Evolution) to comply with rapidly increasing traffic demands.
As one of such attempts, it has been discussed to use not only a frequency band for which a license is granted to an operator (licensed band) but also a frequency band for which a license is not required (unlicensed band) for LTE communication.
In the unlicensed band, in order to avoid interference with another system (such as wireless LAN) different from an LTE system or an LTE system of another operator, a listen-before-talk (LBT) procedure is requested.
The LBT procedure is a procedure in which whether or not a frequency channel (carrier) of the unlicensed band is available is checked, based on the received power (interference power), and only if the channel is confirmed to be a clear channel, the frequency channel is used (See Non Patent Document 1).